Encapsulated ptc heater packages

ABSTRACT

Packaged PTC heater elements wherein the package includes an outer material such as a phenolic resin in the form of a casing and an inner potting material which is thermally conductive and electrically insulated and does not deteriorate the PTC material. The potting material is one that does not include nucleophiles such as amines and the like and therefore excludes the epoxy resins. The potting material used is a silicone resin, polyamides, polyimides and ceramics which are not amine initiated. The PTC device can be completely potted in one of the above noted potting materials or a barrier layer of one of the above potting materials can be placed around the PTC material to separate it from an external potting material and then the PTC material with the barrier layer therearound can be potted with the well known epoxy resin.

United States Patent 1 Ting et a1.

[ ENCAPSULATED PTC HEATER PACKAGES [75] Inventors: Youn H. Ting,Attleboro, Mass; 7 Hans Adolf Stoeckler, Pawtucket,

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

[22] Filed: Oct. 24, 1972 [21] App]. No.: 300,022

[52] US. Cl 174/52 PE, 174/52 S, 219/210, 317/234 E, 317/235 Q, 338/22R, 338/275 [51] Int. Cl. H05k 5/06 [58] Field of Search 219/210, 552;338/275, 338/22, 23, 24; 174/52 S, 52 PE; 317/234, 235 Q; 336/96;29/612; 323/20 [56] References Cited UNITED STATES PATENTS 2,829,3204/1958 Dimond 317/234 3,081,374 3/1963 Burch 174/52 PE 3,210,701 10/1965Fastner et al. 174/52 PE X 3,214,719 10/1965 Turner 338/22 R 3,242,3933/1966 Pauli 338/275X 3,251,015 10/1966 Denham 336/96 3,254,282 5/1966West 174/52 PE X 3,375,774 4/1968 Fujimura. 338/22 R X 3,390,226 6/1968Beyerlein 174/52 PE [11] 3,824,328 [451 July 16,1974

Primary Examiner-Velodymyr Y. Mayewsky Attorney, Agent, or Firm-John A.Haug; James P. McAndrews; Edward J. Connors, Jr.

[57] ABSTRACT Packaged PTC heater elements wherein the package includesan outer material such as a phenolic resin in the form of a casing andan inner potting material which is thermally conductive and electricallyinsulated and does not deteriorate the PTC material. The pottingmaterial is one that does not include nucleophiles such as amines andthe like and therefore excludes the epoxy resins. The potting materialused is a silicone resin, polyamides, polyimides and ceramics which arenot amine initiated. The PTC device can be completely potted in one ofthe above noted potting materials or a barrier layer of one of the abovepotting materials can be placed around the PTC material to separateit'from an external potting material and then the PTC material with thebarrier layer therearound can be potted with the well known epoxy resin.

4 Claims, 2 Drawing Figures PATENIED JUL 1 88974 I 1 ENCAPSULATED PTCHEATER PACKAGES The disclosure relates to a packaging for PTC materials,and more specifically, to a potting material for use with doped PTCmaterials which does not include nuv cleophiles which degrade the PTCmaterial.

In the prior art, epoxy resins have normally been used as pottingmaterials for PTC devices. These potting materials have a tendency todegrade the PTC materials which are normally formed from doped 2ariumtitanate. The reasonfor this is that the epoxy resins are normally madefrom secondary amines or acid anhydrides which are nucleophiles andreact with chemabsorbed oxygen atoms on the boundaries of the PTCdevice. These chemabsorbed oxygen atoms act as electron traps. In theevent that these electron traps become either neutralized or desorbedfrom the grain boundaries, the PTC effect is destroyed in the sense thatelectron traps are no longer available to stop the current flow. Inorder to eliminate this detrimental effect, it is necessary to look forclasses of packaging material's which do not contain nucleophiles.

The degradation takes place in the form of gradual flattening of the PTCresistance-temperature curve. The degradation of theresistance-temperature curve, when the organic polymer, such as theepoxy, acts as a reducing agent, is caused by depletion of the oxygensupply in the grain boundary. In accordance with the present invention,it has been noted that certain potting materials do not cause the abovedescribed degradation of the PTC materials and particularly thedegradation of the resistance-temperature curve. Briefly, in accordancewith the present invention, one embodiment thereof, the prior art epoxypotting materials are replaced by potting materials which do not degradethe PTC heating materials. These include the silicone resin, polyamides,polyimides and ceramics which are not amine initiated. In accordancewith the second embodiment of the invention, the PTC material is coatedwith a barrier of a nondegrading material such as one of the abovedescribed potting materials and then the remainder of the package ispotted with the well known epoxy material.

It is therefore an' object of this invention to provide a PTC heaterpackage with potting material which does not degrade the surface of thePTC heater.

It is a further object of this invention to provide a PTC heater packageusing a potting material taken from the class consisting of siliconeresins, polyamides, polyimides and ceramics which are not amineinitiated.

It is a yet further object of this invention to provide a package forcontaining a PTC heater material wherein the PTC material is surroundedby a nondestructive barrier layer placed between the heater and an epoxyor other degrading potting material.

The above objects and still further objects of the invention willimmediately become apparent to those skilled in the art afterconsideration of the following preferred embodiments, which are providedby way of example and not by way of limitation, wherein:

Referring first to FIG. 1, there is shown a case 1 which may be metallicor may be made out of a material s'uch as phenolic resin. Positionedwithin the case 1 is a PTC heater 3 of well known type to which has beensecured a pair of lead wires 5 and 7. It is understood that the PTCmaterial has been previously metallized in well known manner so that theleads 5 and 7 can be secured thereto by soldering or the like. The wire5 includes insulation therearound which extends outwardly out of thecase I and the wire 7 includes insulation ll therearound which extendsout of the case in the same manner.

In the event that case I is metallic, it is a requirement that the PTCheater 3 be electrically insulated from the case and that the case andpotting compound 13 be thermally conductive. The PTC heater 3 istherefore encapsulated inside the phenolic case 1 by using such FIG. 1is a view in section of a first embodiment of a package in accordancewith the present invention a potting or sealing compound 13 which alsoprovides a tight seal around the PTC pill or heater, thus serving as aheat transfer medium. The potting compound 13 therefore also serves as awater-tight seal for the PTC device 3. In accordance with the presentinvention, the potting compound 13 is free of nucleophiles and istherefore formed from the class of compounds consisting of siliconeresins, polyamides, polyimides and ceramics which are not amineinitiated and which are capable of forming potted compounds of the typede-' scribed above. Some materials of this type which are well known areNovalac epoxy of Dow Chemical Co. which is a silicone rubber. Thesematerials can be used as base potting materials and do not react withthe PTC element, even at elevated temperatures of as high as C. A sealedpotting material 15 can be added on the top of the base potting material13 to provide a better water tight seal.

It can be seen that there is provided a package for a PTC pill or heatermaterial which has the properties of the prior art epoxy and yet doesnot have any material in contact with the PTC heater which can in anyway cause the heater to become degraded.

Referring now to FIG. 2, wherein like character references denote likeelements, there is shown a second embodiment of the invention. Again,there is shown the case 1 and PTC heater 3 which is metallized as-in theprior embodiment and which includes leads 5 and 7 attached thereto withinsulation 9 and 11 surrounding the leads 5 and 7 respectively. However,in accordance with the present embodiment, a layer of material whichdoes not degrade the PTC material 3 and which is labelled 17 completelysurround the PTC element. The entire PTC element 3 with the coating'l7thereon is then placed into thecase l and the entire device with coating17 thereon is now potted with a potting material 19. Since the PTCelement now has an impervious barrier therearound, the potting material19 can be the epoxy resins and the like which are specifically excludedin the first embodiment. The impervious or passivating coating 17 can beformed from silicon resins, RTV silicons, polyimides, polyimide-amide,etc. These materials do not degrade the PTC element in accordance withthe second embodiment.

It has been found that the degradation of the PTC material by thepotting agent is temperature related. It is therefore apparent, thatmany of the potting compounds which can be used in low temperatureapplications are not usable in higher temperature applications.

It is therefore intended that this concept also be included inaccordance with the present invention.

Though the invention has been described with respect to specificpreferred embodiments thereof, many variations and modifications thereofwill immediately become apparent to those skilled in the art. It istherefore the intention that the appended claims be intcrpreted asbroadly as possible in view of the prior art to I include all suchvariations and modifications.

What is claimed is:

l. A PTC heater package which comprises:

a. a PTC device,

b. a potting material encapsulating said PTC device, said pottingmaterial being taken from the class of potting materials which is freeof nucleophiles, said potting material encapsulating said PTC deviceforms a layer contacting and surrounding said PTC device,

c. a further epoxy resin potting material encapsulates said PTC deviceand said potting material,

(1. a casing enclosing said PTC device and potting ma- I 4. A PTC heaterpackage as set forth in claim 3,

wherein said PTC device is formed from doped barium titanate.

2. A PTC heater package as set forth in claim 1, wherein said PTC deviceis formed from doped barium titanate.
 3. A PTC heater package as setforth in claim 1, wherein said potting material is taken from the classconsisting of silicone resins, polyamides, polyimides and ceramics whichare not amine initiated.
 4. A PTC heater package as set forth in claim3, wherein said PTC device is formed from doped barium titanate.